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Natural Hazards HO Pui-sing Contents What are natural hazards Flooding Drought Natural environment Natural environment has provided man with useful resources . Natural environment may be classified into four categories: Physical : energy, mineral and soil resources;

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natural hazards

Natural Hazards

HO Pui-sing

  • What are natural hazards
  • Flooding
  • Drought
natural environment
Natural environment
  • Natural environment has provided man with useful resources.
  • Natural environment may be classified into four categories:
    • Physical: energy, mineral and soil resources;
    • Biotic: forest, fish, animal and other life resources
    • Geomorphic: flat plains and others
    • Locational: good location for transport, communication and defence.
what are natural hazards
What are Natural Hazards
  • The natural environment is unstable.
  • People have been learning to deal with the environmental risks and changes produced by the unstable environment.
  • Extreme natural process (flood, earthquake or landslide…..)has the potential to be a natural hazardif it occurs where people live or have their property (buildings or animals…).
  • Natural disaster when it exceeds normal expectations of frequency or involved.
venn diagram of natural hazards and natural disasters
Venn Diagram of Natural Hazards and Natural Disasters
  • Responding to those hazards, society may seek to modify the natural events system and the human use system of locations, livelihoods, and social organization.
classification of hazards







Blizzard and snow

Avalanche – rock

Fungal disease

Bacterial, viral and protozoal disease

Cold wave

Avalanche – snow

Hay fever





Foot and mouth disease






Expansive soil

Red tide






Shifting sand


Heat wave


Lightning strike and fire

Volcanic eruption

Temperature inversion


Tropical Cyclone


Classification of hazards
measuring and analyzing natural hazards
Measuring and analyzing natural hazards
  • Six indicators to measure and analyze the extreme geophysical events that comprise natural hazards. They are
    • Magnitude
    • Speed of onset
    • Duration
    • Frequency
    • Areal extent
    • Areal reliability
six indicators

It is the most important indicator.

Magnitude rate as ‘extreme’ happenings and pose sufficient threats to the human system to be considered natural hazards.


The Richter Scale

Speed of onset

It refers to the length of time between the first appearance or warning of an event and its peak.


Slow-onset hazards(drought and soil erosion)

Rapid-onset hazards(floods and earthquakes)

Six Indicators
six indicators cont d

It refers to the period of time over which it occurs.


Droughts: seasons or years

Floods: days or weeks

Tornados: minutes or hours


It refers to how often an event of a given magnitude may be expected to re-occur.

‘Return period’: a recurrence interval of 10 years is to say it has in any year a 10% chance of occurring.

Six Indicators – (cont’d)
six indicators cont d10
Areal extent

The area of natural hazards affect.


Avalanche: short and narrow belt of the landscape.

Drought or flood: several thousand km.

Areal reliability

Predictability of an natural hazards occur in an given area.

Predictable and possible for planning.


Volcanic eruptions: fixed point (volcano)

Floods: channels and flood plains

Tropical cyclones: several erratic paths

Earthquakes: unknown

Six Indicators – (cont’d)
impacts of natural hazards and level of economic development
Impacts of natural hazards and level of economic development

Bangladesh (1970) vs United States (1969)

impacts of natural hazards and level of economic development13
Impacts of natural hazards and level of economic development
  • " About 95% of disaster related deaths occur among the two thirds of the world's population that occupy developing countries. In contrast to (these) ... differences in death rate, economic loss from natural disaster is commensurate with income distribution. About three-quarters of absolute global loss occurs in the wealthy countries... (However) the ratio of loss to income is much higher in the developing countries ... In developing countries, disasters may be less frequent but are more catastrophic and more costly in lives and relative wealth, whereas they are increasingly costly in absolute wealth in industrial nations."
  • What is Flooding
  • Nature of flood
  • Casuses of floods
  • Impacts of floods
  • People’s responses to floods
  • Prevention of floods
what is flooding
What is Flooding?
  • Flood:
    • Coastal flooding: (above average sea level)
      • Unusual atmospheric conditions (eg. Onshore hurricane, tornado…)
      • Earthquake or volcanic eruption that set up huge tidal surges.
    • River flooding (flow exceeds bank-full capacity)
      • It is a common hazard and occur anywhere in a river channel at lower course
      • May be caused by
        • Heavy rain
        • Rapidly melting snow
        • Natural or man-made dams collapse
        • others
nature of flood
Nature of flood
  • Floods are the most common of all natural hazards
    • Covers the largest spatial area
    • Causes the greatest loss of life and property
    • Human beings settle on flood plains
      • Abundant water supply
      • Fertile soil for farming
      • Flat relief for development
      • Navigator (transportation)
    • Damage of floods
      • Water inundating (flooding) land, utilities, buildings, crops, communication and transport facilities.
      • After flooding, debris block streets, reservoirs and cover fields.
      • Disruptnormal supply of water, food, shelter and medicine, which cause health and pollution problems.
causes of floods



 Many tributaries

Poor farming methods

 Flood plain  

Over grazing

 Snow melting

Cut ditches to improve the drainage of their moorland

 Heavy rain storms

Erosion of peat

 The land sinking / Sea level rising

No vegetation cover in highland

 Few lakes

Poor mining method to enhance the land sinks

Poor management of embankments

Causes of Floods
impacts of floods




Property Damage


Gujarat, India



Belluno, Italy

> 2000

Vaiont dam overtopped


Pakistan and India


2.27 million crop ha at loss of $63 million


Kavin, Iran

> 2000


Northern Europe

> 2000


Northern China

> 5000


Tianjin, China


Million homeless


Huang He, China


3.6 million affected


Chang Jiang, China



Henan, China


Huang he overflowed, communities destroyed


Huang He, China


Kaifeng city was completely destroyed

Impacts of floods
impacts of floods cont d

1. Loss of life and homeless

2. Crops damaged and loss of livestock

3. Disrupting transport system / network

4. Fertile soil wash away from farmland (soil erosion)

5. Rivers become shallow due to soil deposition (not suitable for navigation)

6. Reduce storage capacity of reservoirs

Impacts of Floods – cont’d

What were the impacts of floods on people and the environment?

people s responses
People’s responses
  • Perceptions
    • How people will interpret various hazards
    • A range of responses
      • Accepting the hazards – Act of God
      • Trying to predict
      • Taking appropriate action to reduce damage potential
      • To offsetting the losses through insurance
people s perception cont d23
People’s perception – cont’d
  • Perception is influenced by the following:
    • The past record of hazards (magnitude and frequency)
    • The strength of traditional cultures (attitudes to the environment)
    • Education standard of the community
    • Wealth and economic development
    • Community awareness and preparations
    • Willingness of local and national government to spend money on
      • Long-term hazard prevention and damage reduction schemes, or
      • Short-term emergency relief after a disaster
people s perception cont d24
People’s perception – cont’d
  • These factorsemphasized the social or cultural framework in which people live.
  • In fact, the range of choices open to people is very limited and controlled by the social, economic and political conditions and pressures.
  • From this perspective, it can explain why people often do seemingly irrational things.
  • Bounded rationality vs Satisfying behaviour
people s perception cont d25
Bounded rationality

People like to make a rational choice of responses.


Few people have access to full information

Many are just not aware of all the alternativeresponses available

People differ in their ability

Few like to forget previous painful history

Satisfying behaviour

People make choices that help them achieve a satisfactory level of reward, but

Stop short of striving for the highest possible level.


Accept ‘tolerable’ levels of hazard

Avoid the worst of the impact

Their property is insured anyway.

People’s perception – cont’d
people s choices
Only those people have a range of choices of response to hazards

Access to full information.

Strong networked of family

Strong friendship support

Strong wealth and political power

People’s Choices
prevention of floods
Prevention of floods
  • Flood management strategies
  • Methods of floods prevention
    • Behavioural
    • Structural
  • Example
flood management strategies
Flood management strategies
  • Individual can do little on the manage the impact of hazards except personal preparations and insurance.
  • Flood management should be a collective action and ought to be coordinated by government (local and national)
    • Most governments only provide emergency relief and reconstructionafter the hazards for facing many conflicting demands on the public purse with limited resources.
    • Economically richer nations, governments have adopted a range of actions to predict, prevent or minimize the effects of hazards and providing disaster relief and reconstruction.
twelve actions to manage the hazards
Twelve actions to manage the hazards
  • 1.Subsidized flood insurance for farmers and


  • 2.Use of existing government laws to ensure local

planners take proper and consistent account of flood hazards.

  • 3.Development of a uniform method of determining flood frequency.
  • 4.An improved system of flood forecasting.
  • 5.Compulsory household flood insurance scheme similar to compulsory third party car insurance.
  • 6.Government support for local council flood control projects, surveys, and engineering works.
twelve actions to manage the hazards cont d
Twelve actions to manage the hazards – cont’d
  • 7. Community education programmes to disseminate flood hazard information and alternative methods of reducing

flood losses.

  • 8.Research to delimit major flood areas and to provide flood hazard information in the form of maps, chart, graphs and

narrative descriptions.

  • 9.Use of zoning regulations to locate low intensity land usesin flood-prone areas.
  • 10.Development of flood warning system, based upon agreed lines of responsibility and communication, using established

flood heights as the determinants of action.

  • 11.Funding of local emergency relief services
  • 12.Funding to support relocations and flood-proofing as

alternatives to repetitive rebuilding.


Management Strategy

Local Council

National Government


3, 4, 8, 10

3, 4, 8

Hazard prevention

2, 6, 8

6, 8

Impact Reduction

1, 5, 7, 9, 10

1, 5, 7

Relief and Reconstruction

11, 12

11, 12

methods of floods prevention

Accepting loss (Third World)

Public relief funds

Flood insurance

Flood forecasting and warning


Reserviors (dams building)

Channel enlargement

Channel straightening


Flood relief channels


Flood plain zoning


Methods of floods prevention
case study thames flood barrier scheme
Case Study: Thames flood barrier scheme
  • Location: Central London
  • Painful events: 1928 (14 people died), 1953 (300 people died)
  • Reasons:
    • Southern Britain is slowly sinking
    • Very slow rise in world sea level
    • Special weather conditions (north-easterly winds from North Sea make a surge to London)
  • Time: October to March (high tides and surges of water)
case study thames flood barrier scheme34
Case Study: Thames flood barrier scheme
  • Scheme: completed in 1982
  • Location: River Thames at Woolwich
  • Construction:
    • Four main gates which can swing up from the river bed to form a continuous steel wall against the incoming flood.
    • The gates lie on the river bed when not in use for ships sailing up and down river.
    • River banks were also raised and strengthened.
  • Warming system:
    • 4 hours before flooding is expected, announcements will be made on TV and radio.
    • 1 hour to London’s flood, sirens will be sounded in riverside areas, and
    • Police will warn people with loud hailers to go to safety places.
  • What is drought?
  • Problems presented to man.
  • Role of man in causing the drought hazard
  • People’s perception and responses to drought
what is drought
What is drought?
  • More than 1/3 of land is dry or very dry.
  • Desert: annual rainfall < 250 mm
  • Definition of Drought:
    • “a period of unusually or unexpectedly low rainfall, which upsets the ecological balance.”
    • A condition in which the amount of water needed for transpiration and direct evaporation exceeds the amount available in the soil.
    • In term of the water need of a particular crop growing under a specific combination of environmental conditions.
  • Three classes of drought can be identified:
    • Permanent drought associated with arid climate.
    • Seasonal drought: annual periods of dry weather
    • Drought due to precipitation variability or unreliable
problems to man
Problems to man
  • Effects of droughts:
    • People themselves and their way of life
    • Crops and livestock
    • Natural vegetation and wildlife
    • Soil
    • Population size and population redistribution
role of man in causing the drought hazard
Role of man in causing the drought hazard
  • The human context in which hazards offer is more important than the geophysical causes of the event.
  • Some factors influencing the human impact of natural hazards
    • Population density in the area affected,
    • Prior experiences of hazards in the area,
    • Traditional methods of coping with hazards
    • The degree of accuracy in predicting the hazards,
    • The effects of any warning, preparation and /or evacuation procedures,
    • The speed and effectiveness of local, national and international emergency and long-term relief services,
    • The overall level of economic development in the area affected
population growth effects
Population Growth Effects
  • Population growth has put increasing pressure on the environment.
  • This pressure increases the risk of human-induced hazards and disasters.
  • For examples
    • The hazard of affecting the balance of world climates by extensive clearance of forest. (Amazon)
    • The hazards of disturbing natural ecosystems and food webs by the clearance of vegetation, the use of chemicals in the atmosphere, and pollution.
    • The hazard of increasing the risk of drought, floods and soil erosion by farming marginal areas particularly in semi-arid regions.